This invention relates to meander line antennas and more particularly to a nested meander line antenna configuration for providing ultra wide bandwidth.
As described in U.S. patent application Ser. No. 10/251,131, filed Sep. 20, 2002 by John T. Apostolos assigned to the assignee hereof and incorporated herein by reference, a wide band meander line antenna is configured to be flush mounted to a conductive surface serving as a ground plane by embedding the meander line components within a conductive cavity surrounded at its top edge by the ground plane. This is done with the antenna looking out of the cavity recessed in the surface. By permitting flush mounting of a meander line antenna, not only can the antenna dimensions be minimized due to the use of the meander line loaded antenna configuration, but in aircraft applications no part of the antenna exists above the skin of the aircraft, thereby to minimize turbulence flow.
Moreover, when adapted to wireless handsets or laptop computers, the depth or thickness of the unit need not be increased when providing a wide band antenna, thus to minimize the overall dimensions of the device. Additionally, the flush mounted meander line antenna when utilized in a roof such as in a car does not result in an unsightly protrusion from the top of the car, but rather is hidden in the recessed cavity. This permits that a vehicle can be provided with a wide band antenna that covers not only cellular frequencies but also the PCS band, 802.11, and GPS frequencies.
Such an embedded antenna is based on the meander line loaded antenna described in U.S. Pat. No. 6,323,814 by John T. Apostolos and assigned to the assignee of incorporated herein by reference. It is noted that in this patent a wide bandwidth miniaturized antenna can be provided through the utilization of planner conductors which are feed through a so-called meander line which involves impedance changes to reduce the physical size of the antenna while at the same time permitting wide band operation.
The plates of the meander line loaded antenna are configured to exist above a ground plane and are spaced therefrom, with a meander line connecting its top plate or element to the ground plane.
Note that the low frequency cut off meander line loaded antennas described in U.S. Pat. No. 6,323,814 and more particularly the meander line loaded antenna described in co-pending patent application Ser. No. 10/123,787, filed Apr. 16, 2002 are assigned to the assignee hereof and are incorporated herein by reference. The low frequency cut off these meander line loaded antennas is decreased due to a cancellation of the reactance of the antenna by the reactance of the meander line and parasitic capacitance.
While the above summarizes the availability of embedded meander line loaded antennas, it will be appreciated that the bandwidth of such antennas is normally no better than 3:1, a ratio of the highest frequency to the lowest frequency of the antenna.
While such an antenna may be made to operate in the 30 to 90 MHz region of the electromagnetic spectrum, there is a requirement to have the bandwidth of the antenna extend between 30 and 500 MHz, a ratio of 18:1.
Moreover, for automotive applications it is sometimes necessary to go from 800 MHz, the cellular band, all the way up to 6,000 MHz for various applications. It will be appreciated that there is no single-cavity meander line loaded antenna which has such an ultra wide bandwidth.
Pushing the upper frequency limit is a problem while maintaining the low frequency cut off. There is a serious problem if one were to try to extend the upper limit of a wide band embedded meander line loaded antenna in terms of its radiation pattern. While the desired radiation pattern from such an antenna would be a loop type pattern or in general omni-directional, when the depth of the cavity is increased to lower the low frequency cut off of the antenna, there is a significant null in the radiation pattern at the higher frequencies which is normal to or perpendicular to the face of the antenna.
Thus, if when one tries to widen the bandwidth of the cavity embedded meander line loaded antenna, as one goes up in the frequency the cavity depth increases. However, with a depth increase one obtains a null in the straight up direction or the direction normal to the plane of the top plate of the antenna.
What this means is that when trying to devise an ultra wide band antenna, the null in the direction perpendicular to the face of the antenna prevents omni directional radiation patterns and thus prevents the antenna from operating properly when it is directly above either a radiating source or when the antenna is used as a transmitting antenna to project energy downwardly in the direction of the null of the antenna pattern.
In order to solve the problem of the null in the orthogonal direction while at the same time providing an exceptionally compact ultra wide band antenna, what one does is to nest and serially connect antenna modules through a common feed, with each module operating in a separate contiguous band to provide continuous coverage. For instance, one antenna module might go from 270 to 500 MHz, where the next module would go from 90 to 270 MHz and a third one from 30 to 90 MHz to provide a 30 to 500 MHz bandwidth. Thus, one way to establish wide bandwidth operation over such a range is to provide three embedded meander line loaded antenna modules working respectively at 30 to 90 MHz, 90 to 270 MHz and 270 to 500 MHz. Note that each of these antenna modules provides a trap so that as one increases frequency. Successive modules come into play by having only the appropriate antenna module radiating energy.
As will be seen, in one embodiment these antenna modules are cavity embedded meander line loaded antennas. In a further embodiment these cavity embedded meander line loaded antennas are nested. This provides a compact miniaturized design with some significant advantages or attributes.
In operation, in the above example when driving this antenna at a frequency between 270 and 500 MHz the first serially-connected module absorbs all the energy, meaning that very little of the energy is transmitted by the 90 to 270 MHz module or the 30 to 90 MHz module. This means that these modules do not contribute to the antenna radiation pattern and thus there is no null.
Moreover, when the frequency goes down from 270 to 90 MHz there is a transition region. For instance, in the transition region half of the power is radiated by the first module, with the second half being radiated by the second module. This occurs at the frequency transition between the adjacent modules. As one moves further lower from 270 MHz all of the energy is radiated by the second module, with the first and third modules radiating little if any energy. In this manner the modules act as antenna traps.
One of the important factors is that as one transits from one module to the next adjacent module one does not want the energy radiated from one module to be out of phase with the energy radiated by the other module. One therefore wants a smooth transition between the modules. What is needed is a geometry associated with modules which accomplishes the transition without frequency domain distortion and this is provided by the subject nesting.
What is therefore provided is a unitary structure that can be made compact and which has a bandwidth defined by the sum of the bandwidths of the nested meander line loaded antenna modules. The nesting removes the problem of driving a given embedded cavity meander line loaded antenna at such a high frequency that a null in the orthogonal direction is created. The nesting of the meander line loaded antenna modules thus provides an ultra bandwidth antenna with a loop like omni directional radiation pattern.
The nesting also minimizes the real estate occupied by such an antenna so that an ultra wide band antenna may be provided embedded into the skin of an aircraft yet still operate over an exceedingly wide frequency band.
More particularly, in order to provide an ultra wide response to a meander line antenna, a series of separate meander line loaded antennas which are cavity embedded are nested, one in side of the other, with the meander lines for the various adjacent bands being coupled to the top ground plates of the antenna, either by capacitive coupling or by direct coupling.
It has been found that by so doing, the effective radiation pattern for the antenna over the entire ultra wide bandwidth is omni-directional or loop like, with any null in the direction normal to the face of the antenna being eliminated.
The bandwidth of such an antenna can be arbitrarily wide depending on the number of nested meander line loaded antenna components that are serially coupled together. Note that all of the antenna modules have a common feed. The use of a number of meander line loaded antenna components nested one within the other, eliminates the orthogonal null that would be created if one were to try to use only one embedded cavity meander line loaded antenna and drive it to higher frequencies. This means that for an original allocation of real estate, for instance, on an aircraft, an antenna may be provided with an exceedingly wide band response through the subject nesting.
In one embodiment, the innermost of the nested cavity embedded meander line loaded antennas has a portion of its meander line capacitively coupled to an overlying ground plane plate. The next outer cavity embedded meander line loaded antenna has a portion of its meander line also capacitively coupled to the ground plane plate. The last of the nested cavity embedded meander line loaded antennas has its upper most meander line portion directly coupled to the ground plane plate.
In one embodiment, the common feed for such a nested arrangement goes up through the nested cavities, and when balanced is coupled across the innermost portions of opposed ground planes.
While there are critical military requirements for ultra wide band flush mounted antennas for use on aircraft, military vehicles and the like, the ultra wide bandwidth of such antennas is also critical in wireless devices including wireless LANS, laptop antennas and all manner of multi-band operation including ultra wide band transmissions.
In summary, a nested cavity embedded loop mode antenna is provided with an ultra wide band response by nesting individual embedded cavity meander line loaded antenna modules, with the meander lines coupled to a ground plane plate either capacitively or directly so as to provide as much as a 27:1 ratio of high frequency to low frequency cutoff. The nested meander line structure is exceptionally compact and eliminates the problem of a null in the antenna radiation pattern perpendicular to the face of the antenna, thus to provide a loop type antenna pattern at all frequencies across which the antenna is to be operated. The use of the nested meander line configuration provides a flush mount for the antenna having a footprint associated with the larger of the meander line cavities and thus the lowest frequency of operation, the nesting precluding the necessity of providing separate side-by-side meander line loaded antennas which would increase the real estate required.